Yes, the connector is for a ESP8266 01 module. I put it in on an early prototype and then we decided to keep it. Unfortunately I've never had the time to implement any software for it. It's connected to one of the serial ports on the STM so it should be possible to talk to the module though it.

Libraries for using the UART on the STM32 are in the library from ST. You can find the API for the UART here. Initially we used an FTDI cable before the USB was implemented and this code is still in, but removed at compile time with the macro USE_FTDI_UART. Search for that in the code and you'll get an idea of how to use the UART.